This invention relates to improvements in double-acting, fabric covered, power transmission belts.
Double-acting power transmission belts have been known for many years. Such belts have been described in U.S. Pat. No. 3,673,883 (Adams), U.S. Pat. No. 3,338,107 (Kiekhaefer), U.S. Pat. No. 3,394,968 (Cicognani), U.S. Pat. No. 2,699,685 (Waugh), and French Pat. No. 1,556,839 (Pirelli).
To obtain maximum belt life in a synchronous belt, the teeth must be sufficiently rigid to carry the load and yet be able to deflect so as to mate properly with the grooves of the pulley. Since the internal strength and rigidity of the tooth elastomer is not sufficient to carry the load by itself and certainly does not itself exhibit sufficient abrasion resistance, a fabric cover is conventionally bonded on the surface of the belt. Historically, the fabric cover which has been employed was made from a so-called "stretchable" nylon fabric. Using stretchable nylon has made the production of such belts economical, using the method described in U.S. Pat. No. 3,078,206 (Skura). According to the above listed patents, double-acting belts, i.e. belts having teeth on both the inner and the outer peripheral surfaces thereof, are also conventionally made with stretchable fabric jackets and, in one case, namely the French Pat. No. 1,556,839, with a stretchable fabric jacket on only the inside and no jacket on the outside teeth.
It is also known, as stated for example in U.S. Pat. No. 4,514,179 (Skura), to make conventional one-sided toothed belts with a so-called "non-stetchable" fabric jacket. Such belts having a non-stretchable fabric jacket, as described in the latter patent, result in a belt having longer belt life and greater horse-power capability than similar belts with a jacket of stretchable fabric. The reason for this is believed to be that the conventional stretchable nylon jacket, which is made with loosely twisted crimped yarn, has its interstices substantially stretched and filled with tooth rubber during the belt molding process. Vulcanization locks the then stretched fabric in place causing it to become stiff and consequently reducing its ability to deform under load. Under operating loads the belt teeth are, however, forced to deflect and subject the thusly stiffened fabric cover to excessive dynamic stresses. This results in premature rupture of the rubber-filled fabric cover, particularly in the tooth root region, and early belt failure.
It is believed that in belts having a jacket of stretchable fabric, approximately 80% of the load on the belt tooth is transmitted by the fabric and only approximately 20% is transmitted by the rubber tooth itself. This ratio concentrates excessive load on the fabric, causing the early belt failure which has been experienced with such belts.
Double-acting power transmission belts have been constructed traditionally by using stretchable fabric for jacketing the teeth and land areas between the teeth on both working sides of the belt. If, however, approximately 80% of the load on the belt tooth is transmitted by the fabric and only approximately 20% by the rubber tooth itself, this characteristic becomes even more of a problem with double-sided, i.e. double-acting, power transmission belts than with single sided belts. In double-acting belts the belt is usually driven by a toothed drive pulley which engages the inner side, i.e. the inner peripheral surface, of the belt. At least one driven pulley also engages the same inner side of the toothed belt, while one or more additional driven pulleys engage the outer toothed surface of the double-sided toothed belt. Thus, it will be seen, in such double-sided belt systems, the drive pulley is in engagement with the inner toothed surface while at least one of the driven pulleys is in engagement with the outer toothed surface of the belt. Forces must, therefore, be transmitted from the teeth on the inner toothed surface to the teeth on the outer toothed surface while maintaining accurate synchronization between the teeth. Moreover, the "wrap" of the belt around the inner pulleys is commonly substantially greater than the "wrap" around the outer pulleys in such systems. Also, these drive systems are such that the torque exerted by the drive pulley on the inner side is substantially greater than the torque on any given driven pulley on the outer side of the belt. The prior art belts failed to provide a belt having characteristics which complement the foregoing system features.
It is an object of the present invention, therefore, to maximize, in a double-acting power transmission belt, the portion of the force transmitted by the elastomeric material of the tooth rubber of the teeth on the inner surface of the belt, through the body of the belt, and thence to the teeth on the outside of the belt.
It is another object of the present invention to provide a double-acting belt which will be able to transmit a greater load than the conventional double-acting belts, between a drive pulley at the interior, a driven pulley at the interior and a driven pulley at the exterior of such belt, while exhibiting increased belt life, or at least maintaining belt life substantially the same as that of conventional double-acting belts.
It is still another object of the invention to provide a belt which exhibits greater uniformity, greater flexibility and requires less power consumption, than the double-toothed belts of the prior art.
It is a concomitant object of the invention to provide a belt construction for double-acting belts which overcomes drawbacks of prior constructions and in which the water-resistant jacket material cooperates with the elastomeric belt of the belt teeth to provide a belt capable of use for relatively high horse power applications while exhibiting a belt life longer than can be expected with known double-acting belts under similar operating conditions, while at the same time exhibiting indexing and uniformity characteristics, between the teeth on opposite sides of the belt, which are superior to those of the conventional double-acting belts.